o Currently a request queue has got fixed number of request descriptors for
sync and async requests. Once the request descriptors are consumed, new
processes are put to sleep and they effectively become serialized. Because
sync and async queues are separate, async requests don't impact sync ones
but if one is looking for fairness between async requests, that is not
achievable if request queue descriptors become bottleneck.
o Make request descriptor's per io group so that if there is lots of IO
going on in one cgroup, it does not impact the IO of other group.
o This patch implements the per cgroup request descriptors. request pool per
queue is still common but every group will have its own wait list and its
own count of request descriptors allocated to that group for sync and async
queues. So effectively request_list becomes per io group property and not a
global request queue feature.
o Currently one can define q->nr_requests to limit request descriptors
allocated for the queue. Now there is another tunable q->nr_group_requests
which controls the requests descriptr limit per group. q->nr_requests
supercedes q->nr_group_requests to make sure if there are lots of groups
present, we don't end up allocating too many request descriptors on the
queue.
Signed-off-by: Nauman Rafique <nauman google com>
Signed-off-by: Vivek Goyal <vgoyal redhat com>